"The book says it is the ‘Property of the Half-Blood Prince,' but there is no name and no other record of him, so his identity is an enigma. But whoever he was, he was obviously very smart; he was capable of taking the conventional recipes for certain potions and spells and making them significantly better. He was an original thinker but also quite a dark thinker."

I totally agree, I could never quite put my finger on how intelligent Snape is until HBP came how...Snape is incredible...